Diabetes is an adult disease such that reduced action of insulin extremely increases the concentration of glucose in the blood (blood sugar level), and is often accompanied by complications such as heart diseases, cerebral infarction, foot gangrene, and blindness caused by retinal detachment. The Ministry of Health, Labour and Welfare survey of diabetes in 2002 reports that approximately 7.4 million people in Japan are “strongly suspected of” diabetes, and the number of the potential diabetics “who may have diabetes” amounts to approximately 16.2 million, which corresponds to one adult per 6.3 adults. In the future, further increase in the number of the diabetes is expected not only in Japan but also in the world. Usually, it is difficult to become aware of diabetes until the concentration of glucose is extremely increased or serious complications are demonstrated. Accordingly, a periodical diagnosis at an early stage including a blood test is particularly important for prevention of the diabetes.
Usually, the blood test is performed in order to monitor a blood sugar level in the blood in real time. For this, a skin of a person to be tested needs to be pierced by a needle to extract the blood. Such extraction of the blood, however, gives much pain to the person to be tested, and includes a risk that the person to be tested may be infected with various infectious diseases unless the needle is treated safely.
For this, establishment of a non-invasive method for accurately measuring the concentration of glucose in the blood without extracting the blood has been strongly desired.
Several non-invasive methods for measuring the blood sugar level have been proposed. For example, a method has been proposed in which the concentration of glucose in an object to be measured is determined using near-infrared spectroscopic analysis (Patent Literature 1). In the method, near-infrared radiation is projected onto the skin of a person to be tested, and the emitted light from the skin is received by an optical fiber bundle.
The spectrum of the emitted light by the optical fiber bundle is analyzed to detect absorption signals from a first wavelength band having an absorption peak of an OH group derived from the glucose molecules (for example, 1550 to 1650 nm), a second wavelength band having an absorption peak of an NH group derived from the glucose molecules (for example, 1480 to 1550 nm), and a third wavelength band having an absorption peak of a CH group derived from the glucose molecules (for example, 1650 to 1880 nm). The concentration of glucose is determined by multivariate analysis based on these absorption signals.
Moreover, a method has been proposed in which the concentration of a target component in a medium is determined based on a probability and statistics simulation (Patent Literature 2). In the method, a group of optical paths in the medium is analyzed by the probability and statistics simulation such as a Monte Carlo method. A database is created, which shows an absorption coefficient as an optical property of the medium and change of diffusion reflectance in the case where an equivalent diffusion coefficient is changed within a predetermined range thereof. Next, the diffusion reflectance is smoothed by a method of regression analysis to create a correction database. Next, the medium is irradiated with near-infrared light in the wavelength band of 1000 to 2500 nm to detect the emitted light from the medium, and the thus-obtained measured spectrum is compared with the reference spectrum provided by the correction database to determine the concentration of the target component in the medium. According to the method, if the change in the spectrum caused by change in the concentration of the component other than the target component in the medium is computed from the correction database, the concentration of the target component can be determined from the measured spectrum by the multivariate analysis such as principal components regression (PCR) and multiple regresssion analysis (MLR).